During the imaging of a sample in a particle-optical device, such as e.g. a scanning electron microscope (SEM), the sample is irradiated by a beam of charged particles—by electrons in this case. Interaction products such as e.g. secondary electrons are released as a result of the interaction between the particles of the primary particle beam and the sample, the interaction products being detected and used for generating an image of the sample.
Samples which are imaged in this way should usually be electrically conductive. This allows the electric charge applied to the sample by the irradiation with electrons to dissipate. The same also applies if the sample is imaged not by an electron microscope but with the aid of an ion microscope and the sample is accordingly irradiated by ions. In both cases, charge applied to the sample surface is prevented from dissipating if non-conductive sample materials are used, and so there is a collection of electric charge on the sample.
Such electrostatic charging generally has an interfering effect on the image recording since it leads to a local signal change which falsifies the information content of the recorded image. Moreover, there can be unwanted migration of the image or distortions of the image. The release of secondary electrons from the sample can also be reduced, leading to a reduction in the image contrast.
Charging artifacts may also occur if dirt particles are present on a sample which is electrically conductive per se, the dirt particles behaving like local insulators such that the dirt particles charge and cause signal amplifications which interfere with the image impression.
During the observation in the SEM, the charging of the sample can be positive or negative. The sign of the charging depends, inter alia, on the landing energy, with which the primary electrons are incident on the sample, and on the local composition of the sample.
If the primary electrons are incident with an energy that is lower than a first critical energy depending on the composition and nature of the sample, the number of primary electrons exceeds the number of secondary electrons released from the sample material, and so there is negative charging. Negative charging may also occur if the primary electrons have a landing energy that is higher than a second critical energy which in turn is determined by the composition and nature of the sample. If the second critical energy is exceeded, primary electrons penetrate deep into the sample, and so only a few secondary electrons are released from the sample surface.
By contrast, if the kinetic energy of the primary electrons is selected in such a way that more secondary electrons are released than are replaced by incident primary electrons, the sample experiences positive charging.
When imaging with the aid of a focused ion beam (FIB), which usually consists of positive ions such as Ga+ or He+ ions, there may be an excess of positive charges as a result of the incidence of the primary particle beam.
In order to obtain particle-optical images with an improved quality and improved information content, it is desirable, as a matter of principle, to prevent, compensate or at least reduce electric charging of the sample.
Various methods and devices have been described, by which it is possible to counteract the aforementioned, unwanted charging artifacts.
It is possible to make the sample electrically conductive, or improve the conductivity thereof, by applying a coating made of conductive materials such as metals or carbon onto the sample surface. A disadvantage of this is that the sample is prepared with much outlay prior to the observation in the particle-optical device. Moreover, coating the sample may be generally undesirable.
Furthermore, a method in which the sample is observed during low vacuum operation is known. A disadvantage of this is that a special low-vacuum microscope with a specifically designed vacuum system and a specific detection system is involved.
In the case of electron microscopes which use electrostatic immersion lenses, i.e. objective lenses in which the sample lies within the objective lens and a potential difference dominates between the sample and the pole shoes of the objective lens, there moreover can be arcing between the pole shoe of the objective lens and the sample in the case of a vacuum that is too low.
For work with ion microscopes in particular, it has been proposed, in order to avoid positive charging, to additionally irradiate the sample with electrons in addition to the ion irradiation in order to neutralize the positive charges by negative charges. To this end, the employed ion microscope has an additional electron-optical column (flood gun). However, this method is unsuitable for electrostatic immersion lenses since the extraction fields present in this case accelerate the charged particles, and so, inter alia, charging effects would even be amplified.
A further method is based on the ionization of gases. In this case, gas is introduced in the vicinity of the sample such that the incident primary particle beam or the emitted secondary electrons interact with gas molecules. As result, charged gas particles and additional electrons can be created, which reduce or completely compensate the charging on the sample surface. A disadvantage in this case is that the overlap of the electron beam with the gas molecules, i.e. the simultaneous presence of electron beam and gas molecules, needs to be as short as possible. Otherwise, the secondary electrons interact with the charged gas particles and therefore they can no longer be used to generate an image, leading to deterioration in the image quality. Therefore, this means that gas needs to be introduced in the direct vicinity of the sample surface, and so the components used to this end are generally arranged between objective lens and sample, which may have a negative effect on the operation of the particle-optical device. Moreover, an excess of gas in the vacuum system of the device may have a disadvantageous effect on vacuum pumps and a particle source.
The following documents should be considered:                DE19851622A1        EP1455379        DE 3332248A1        DE10 2012 001 267 A1        